STRUCTURE OF SKELETAL MUSCLE Dr.Mohammed Sharique Ahmed Quadri Assistant Professor, Physiology م ي ح ر ل ا ن م ح ر ل ه ا ل ل م ا س ب
Dec 21, 2015
STRUCTURE OF SKELETAL MUSCLE
Dr.Mohammed Sharique Ahmed QuadriAssistant Professor, Physiology
بسم الله الرحمن الرحيم
Objectives
By the end of this lecture, you should be able to:
Draw and label a skeletal muscle at all anatomical levels, from the whole muscle to the molecular components of the sarcomere. At the sarcomere level, include at least two different stages of myofilament overlap.
Draw Diagram the structure of the thick and thin myofilaments and label the constituent proteins.
Describe the functional importance of the subunits.
Categorization of Muscle
Structure of Skeletal Muscle
• Skeletal muscle consists a number of muscle fibers lying parallel to one another and held together by connective tissue
• Single skeletal muscle cell is known as a muscle fiber– Multinucleated – Large, elongated, and cylindrically shaped– Fibers usually extend entire length of muscle
Structure of skeletal muscle
Structure of Skeletal Muscle
• Myofibrils–Contractile elements of muscle fiber–Regular arrangement of thick and thin
filaments• Thick filaments – myosin (protein)• Thin filaments – actin (protein)
Viewed microscopically myofibril displays alternating dark (the A bands) and light bands (the I bands) giving appearance of striations
Structure of Skeletal Muscle• Sarcomere
– Functional unit of skeletal muscle– Found between 2 Z lines (connects thin filaments of two
adjoining sarcomeres)– Regions of sarcomere
• A band–Made up of thick filaments along with portions of thin
filaments that overlap on both ends of thick filaments• H zone
– Lighter area within middle of A band where thin filaments do not reach
• M line– Extends vertically down middle of A band within center
of H zone• I band
– Consists of remaining portion of thin filaments that do not project into A band
Cytoskeleton components of myofibril
Myosin • Component of thick filament• Protein molecule consisting of two identical
subunits shaped somewhat like a golf club– Tail ends are intertwined around each other– Globular heads project out at one end
• Tails oriented toward center of filament and globular heads protrude outward at regular intervals– Heads form cross bridges between thick and thin
filaments• Cross bridge has 2 important sites critical to contractile
process– An actin-binding site– A myosin ATPase (ATP-splitting) site
Structure and Arrangement of Myosin Molecules Within Thick Filament
Actin
• Primary structural component of thin filaments• Spherical in shape• Thin filament also has 2 other proteins– Tropomyosin – Troponin
• Each actin molecule has special binding site for attachment with myosin cross bridge – Binding results in contraction of muscle fiber
Composition of a Thin Filament
• Actin and myosin are often called contractileProteins.– Neither actually contracts.
• Actin and myosin are not unique to muscle cells, but are more abundant and more highly organized in muscle cells.
Tropomyosin and Troponin
• Often called regulatory proteins • Tropomyosin– Thread-like molecules that lie end to end alongside
groove of actin spiral– In this position, covers actin sites for binding with myosin
, blocking interaction that leads to muscle contraction• Troponin – Made of 3 polypeptide units
• One binds to tropomyosin• One binds to actin• One can bind with Ca2+
Tropomyosin and Troponin• Troponin–When not bound to Ca2+
• Troponin stabilizes tropomyosin in blocking position over actin’s cross-bridge binding sites
–When Ca2+ binds to troponin• Tropomyosin moves away from blocking
position–With tropomyosin out of way, actin and
myosin bind, interact at cross-bridges• Muscle contraction results
Cross-bridge interaction between actin and myosin brings about muscle contraction by means of the sliding filament mechanism.
T Tubules and Sarcoplasmic Reticulum
Sarcoplasmic Reticulum
• Modified endoplasmic reticulum• Consists of fine network of interconnected
compartments that surround each myofibril• Not continuous but encircles myofibril
throughout its length• Segments are wrapped around each A band
and each I band– Ends of segments expand to form saclike regions –
lateral sacs (terminal cisternae)
Transverse Tubules
• T tubules• Run perpendicularly from surface of muscle cell
membrane into central portions of the muscle fiber• Since membrane is continuous with surface
membrane – action potential on surface membrane also spreads down into T-tubule
• Spread of action potential down a T tubule triggers release of Ca2+ from sarcoplasmic reticulum into cytosol
Relationship Between T Tubule and Adjacent Lateral Sacs of Sarcoplasmic Reticulum
References
• Human physiology by Lauralee Sherwood, 7th edition
• Text book physiology by Guyton &Hall,12th edition
• Text book of physiology by Linda .s contanzo,third edition
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